Just as a footnote to this discussion, Win 32-bit CS5 didn't see my Epson V700 scanner in TWAIN mode (WIA mode only) until I downloaded the "additional plug-ins" packet from Adobe and copied the TWAIN plug-in to CS5's Import-Export plug-ins folder.

TWAIN mode invokes the very nice Epson Scan software from within the PS import function. WIA gives you a very limited little scan applet instead.